Structure and types of neuron (The nervous tissue)

February 3, 2018 Sushil Humagain Anatomy and Physiology, Biology 0

  • The nervous tissue is purely ectodermal in origin and has the least regeneration power.
  • It first appeared in the members of Coelenterata (Cnidaria) in which the nerve cells are non-polarized.
  • The structural and functional unit of the nervous tissue is a neuron or nerve cells.
  • The total number of neurons in the human nervous system is about 1012e. 1 trillion.

Neuron:

  • Neuron is the structural and functional unit of our nervous system.
  • It is an electrically excitable cell that receives, processes, and transmits information through electrical and chemical signals called the nerve impulse.
  • There are 3 types of neurons.

  1. Unipolar or pseudo-unipolar neuron: In such neurons, single process arises from the cyton or cell body and bifurcates to form axon and dendrite. E.g. neurons of dorsal root ganglion of the spinal cord.
  2. Bipolar neuron: In such neurons, two processes, one dendrite and one axon, arise from the cell body. E.g. neurons of retina and internal ear.
  3. Multipolar neuron:
  • Such neurons have one axon and two or more dendrites.
  • More than 99% of the neurons are multipolar type.
  • The neurons are surrounded by various types of supporting cells or packing cells called Neuroglia.
  • The number of glia cells is 30-40 times the number of neurons.
  • The glia cells around the neurons of the peripheral nervous system are called Schwann cells, while the glia cells of the central nervous system are of three different types:
    1. Astrocytes: Star shaped cells forming blood brain barrier for the transportation of nutrients into the brain (CNS).
    2. Microglia: Phagocytic cells that remove debris from the nervous tissue.
    3. Oligodendrocytes: Secrete myelin-sheath around many nerve fibers together.

Structure of a neuron:

  • A typical multipolar neuron consists of three parts: Dendrites (Dendrons), Cell body (Cyton) and Axon.
  • In polarized fibers, the nerve impulse travels from dendrites to the cyton and from cyton to the axon.
  • Cyton or cell body contains groups of ribosomes and rough endoplasmic reticulum called Nissl granules.
  • Such granules are present in the dendrites but absent in the axon.
  • The presence of Nissl granules is the characteristic feature of neurons.
  • The centrosome is absent in the cyton and hence the neurons cannot divide.
  • The cytoplasm and the cell membrane of the axon are called axoplasm and axolemma
  • The long fibers of i.e axon or dendrite may be gray (non-myelinated) or white (myelinated).
  • The myelin sheath in PNS is secreted by Schwann cells. Myelinated nerve fibers are also called medullated
  • The covering formed by Schwann cells around the axon is called Neurilemma.
  • Due to discontinuous myelin sheath around the myelinated axon, the fibers are differentiated into nodes (nodes of Ranvier) and internodes.
  • At nodes, the myelin sheath is absent but neurilemma is present.
                          A typical myelinated multipolar neuron

Ganglion: A ganglion is a group of cytons or cell bodies. In brain (CNS), such groups are called nuclei.

Nerves: A nerve is mainly the group of axons and in brain (CNS), such group of axons is called a tract.

Structure of a nerve:

  • The outermost connective tissue covering of the nerve is called an epineurium.
  • There are several bundles of neurons embedded in the epineurium which are called nerve fasciculi.
  • Each nerve fasciculus has a connective tissue covering called perineurium.
  • Individual neuron is also surrounded by a covering of connective tissue called endoneurium.
  • Outer to inner, connective tissue covering in a nerve is:

Epineurium———> Perineurium ——–> Endoneurium

Types of nerves:

On the basis of their functions, the nerves are of following three types:

  1. Sensory nerves or afferent nerves: They carry the nerve impulse from the receptor organs or sense organs (skin, retina, nose, ear etc.) to the brain or spinal cord (CNS). E.g. Optic nerve, Auditory nerve etc.
  2. Motor nerves or efferent nerves: They carry the nerve impulse from the brain or spinal cord (CNS) to the effector organs (muscle or glands). E.g Trochlear nerve etc.
  3. Mixed nerves: In such nerves, some fibers carry the impulse towards the CNS and the others carry away from the CNS .i.e both sensory and motor nerve fibers are present in them. E.g Vagus nerve etc.

Structure and types of neuron (The nervous tissue)